Displacement sensor with temperature compensation by combining outputs in a predetermined ratio

ABSTRACT

A displacement sensor provided with a magnet (4) and magnetoelectric conversion element for sensing the variation of magnetism caused by the magnet to sense the displacement of the magnetic material to be measured, which includes a first magnetoelectric conversion element (5) located in the place where the magnetic flux density is varied by the displacement of the material to be measured (7), a second magnetoelectric conversion element (6) of the same kind as the first magnetoelectric conversion element (5) located in the place where the magnetic flux density is not varied by the displacement of the material to be measured, and a circuit element (10) for adding the outputs of the first and second magnetoelectric conversion element at a predetermined ratio of V20/V10, wherein V20 and V10 are outputs from the first and second magnetoelectric conversion elements, respectively, for a displacement of 0 at a temperature of 20° C.

FIELD OF THE INVENTION

The present invention relates to a displacement sensor which comprises amagnet and magnetoelectric conversion means for sensing the variation ofmagnetism caused by the magnet, and senses the displacement of amagnetic material to be measured, such as iron.

BACKGROUND OF THE INVENTION

Sensing the location of a magnetic material or magnet by usingmagnetoelectric conversion elements such as Hall elements or magneticresistance elements has been a widely practiced method, for example, insensing the phase of a DC brushless motor. However, even a GaAs Hallelement, which usually has a small temperature coefficient, has atemperature coefficient of -0.06%/°C. during constant-current drive.Therefore, the displacement could not be accurately sensed in a widetemperature range.

In order to overcome this problem:

1) Japanese Patent No. 1094798 (Japanese Laid-Open Patent ApplicationNo. 38073/1981) provides a compound magnetoelectric conversion meanswhich is composed of Hall elements and a temperature sensing element allclosely placed on a base material in the same pattern, and which isthereby capable of performing excellent temperature compensation evenfor local temperature variations in the element itself.

2) In Japanese Laid-Open Patent Application No. 171879/1983, thevariations of output voltage caused by the temperature variations of theHall elements are controlled by forming compensation resistors in serieson a base member.

3) Japanese Laid-Open Patent Application No. 248010/1989 is aimed atobtaining a high precision throttle sensor that is independent oftemperature variations, with the polarities of the temperaturecoefficients of the magnet and the Hall elements opposite to each other.

Although the compound magnetoelectric conversion elements describedin 1) and 2) are excellent in temperature compensation, the productionof these compound magnetoelectric conversion elements is difficult andcostly.

Furthermore, these compound magnetoelectric conversion elements are notgenerally used, and therefore hard to come by.

In a displacement sensor comprising a permanent magnet andmegnetoelectric conversion elements as described in 1) and 2), thesurface magnetic flux density of the permanent magnet itself hastemperature characteristics, and therefore, compensation only for thetemperature characteristics of the magnetoelectric conversion elementsis not enough to compensate for the temperature characteristics of thewhole displacement sensor.

In the case of 3), the problem is the difficulty in adjusting thetemperature coefficients of the magnet and the Hall elements so thatthey are opposed to each other. Though Si Hall ICs are used in place ofHall elements to provide a switching sensor, the electron mobility of Siis so small that the swicthing sensor is not suitable for a highprecision sensor. Further, Si Hall ICs have a large offset voltage,which causes variations in temperature characteristics.

Thus, the displacement sensor the prior art is not capable of performingaccurate sensing due to temperature variations and is not suitable forthe use in a wide temperature range.

Accordingly, the present invention is basically aimed at providing adisplacement sensor that is capable of accurately sensing thedisplacement in a wide temperature range by compensating for thetemperature coefficients of a permanent magnet and magnetoelectricconversion elements. Even when the displacement sensor is used in aplace where the temperature varies greatly, the displacement is sensedwith high precision by the displacement sensor.

SUMMARY OF THE INVENTION

A displacement sensor of the present invention comprises a magnet andmagnetoelectric conversion means for sensing the variation of magnetismcuased by the magnet to sense the displacement of the magnetic materialto be measured. In this displacement sensor, a first magnetoelectricconversion means is located in the place where the magnetic flux densityis varied by the displacement of the magnetic material to be measured,while a second magnetoelectric conversion means of the same type as thefirst magnetoelectric conversion means is located in the place where themagnetic flux density is not varied. A circuit means for adding theoutputs of the first and second magnetoelectric conversion means at apredetermined ratio is also provided in the displacement sensor. Thisstructure is aimed at minimizing the temperature coefficient of thedisplacement sensor in a wide temperature range, wherein the temperaturecompensation is performed for the first magnetoelectric conversion meansby adding at a predetermined ratio the output of the firstmagnetoelectric conversion means located in the place where the magneticflux density is varied by the displacement of the material to bemeasured and the output of the second magnetoelectric conversion meanslocated in the place where the magnetic flux density is not varied.

A displacement sensor of another structure also comprises a magnet andmagnetoelectric conversion means for sensing the variation of magnetismcuased by the magnet to sense the displacement of the magnetic materialto be measured. Provided in this displacement sensor are a firstmagnetoelectric conversion means located in the place where the magneticflux density is varied by the displacement of the magnetic material tobe measured, a second magnetoelectric conversion means of the same typeas the first magnetoelectric conversion means located in the place wherethe magnetic flux density is not varied by the displacement of themagnetic material to be measured, a circuit means having a temperaturecoefficient of the opposite polarity to that of the magnet and themagnetoelectric conversion means, and a circuit means for adding theoutputs of the first and second magnetoelectric conversion means at apredetermined ratio. This structure of the displacement sensor is aimedat further minimizing the temperature coefficient of the displacementsensor, wherein temperature compensation is performed by adjusting thetemperature coefficient of the above circuit means so that it becomesopposite to that of the magnet and the magnetoelectric conversion means.

A displacement sensor of yet another structure also comprises a magnetand magnetoelectric conversion means for sensing the variation ofmagnetism cuased by the magnet to sense the displacement of the magneticmaterial to be measured. Provided in this displacement sensor are afirst magnetoelectric conversion means located in the place where themagnetic flux density is varied by the displacement of the magneticmaterial to be measured, a second magnetoelectric conversion meanslocated in the place where the magnetic flux density is not varied bythe displacement of the magnetic material to be measured, and for addingthe outputs of the two magnetoelectric conversion means at apredetermined ratio. The temperature coefficient of the firstmagnetoelectric conversion means is adjusted so as to differ from thatof the second magnetoelectric conversion means. The first and secondmagnetoelectric conversion means are composed of elements each having atemperature coefficient that is varied depending on the magnetic fluxdensity. Temperature compensation is performed by combining drivingmethods for the first and second magnetoelectric conversion means,thereby further minimizing the temperature coefficient of thedisplacement sensor.

A displacement sensor of still another structure comprises a magnet andmagnetoelectric conversion means for sensing the variation of magnetismcuased by the magnet to sense the displacement of the magnetic materialto be measured. Provided in this displacement sensor are a firstmagnetoelectric conversion means located in the place where the magneticflux density is varied by the displacement of the magnetic material tobe measured, a second magnetoelectric conversion means of the same typeas the first magnetoelectric conversion means located in the place wherethe magnetic flux density is not varied by the displacement of themagnetic material to be measured, and a circuit means for adding theoutputs of the two magnetoelectric conversion means at a predeterminedratio to sense the displacement of the material to be measured, all ofwhich are contained in a case. The case has a heat insulating structureso that the temperature in the case is kept constant independently ofthe temperature of the external environment. Optionally, by using a caseof a material having a high thermal conductivity, the displacementsensor can respond rapidly to the temperature of the externalenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of the sensing element of thedisplacement sensor in the first example of the present invention.

FIGS. 2A and 2B are diagrammatic views of the principle of sensing thedisplacement of the material to be measured.

FIGS. 3A and 3B are graphical views of the output characteristics of themagnetoelectric conversion means in relation to the displacement of thematerial to be measured.

FIG. 4 is a circuit diagrams of an example of the bias cancellingcircuit.

FIG. 5 is a graphical representation of the characteristics of theoutput adjustment by the bias cancelling circuit.

FIG. 6 is a circuit diagram of the circuit element of the displacementsensor.

FIG. 7 is a circuit diagram of another embodiment of the circuit elementof the displacement sensor.

FIG. 8 is a longitudinal sentional view of another embodiment of thesensing element of the displacement sensor.

FIG. 9 is a circuit diagram of the circuit element of the displacementsensor in accordance with the third example of the present invention.

FIG. 10 is a longitudinal sectional view of the sensing element of thedisplacement sensor in accordance with the fourth example of the presentinvention.

FIG. 11 is a graphical representation of the characteristics of thetotal temperature coefficient of the magnet and the Hall elements inrelation to the magnetic flux density.

FIG. 12 is a graphical representation of the relationship between themagnetic flux density and the distance from the magnet to the Hallelements.

FIG. 13 is a longitudinal sectional view of yet another embodiment ofthe sensing element.

FIG. 14 is a diagrammatic view of an example of the constant-voltagedriving circuit for the Hall elements disposed in the displacementsensor in accordance with the fifth example of the present invention.

FIG. 15 is a diagrammatic view of an example of the constant-currentdriving circuit for the Hall elements disposed in the displacementsensor.

FIG. 16 is a schematic view of the structure of the displacement sensorin accordance with the sixth example of the present invention.

FIG. 17 is a graphical view of the output variation ratios of the Hallelements with the lapse of time, wherein the case of the sensing elementof the displacement sensor is made of aluminum in accordance with theseventh example of the present invention.

FIG. 18 is a graphical view of the output variation ratios of the Hallelements with the lapse of time, wherein the case of the sensing elementof the displacement sensor is made of fluorine contained resin.

FIG. 19 is a graphical view of the output variation ratios of thedisplacement sensor with the lapse of time in both cases where the caseof the sensing element of the displacement sensor is made of aluminumand where the case is made of fluorine contained resin.

FIG. 20 is a table showing the circuit output V0 generated when thetemperature is 20° C. or -30° C. and the displacement is 0 mm, 10 mm, or20 mm.

FIG. 21 is a table showing the circuit output V0 generated when thetemperature is 20° C. or -30° C. and the displacement is 0 mm, 10, or 20mm.

FIG. 22 is a table showing the circuit output V0 generated when thetemperature is 20° C. or -30° C. and the displacement is 0 mm, 10, or 20mm.

FIG. 23 is a table showing the relationship between the output V0 andthe outputs V1 and V2 when the displacement is 10 mm.

FIG. 24 is a table showing the circuit output V0 generated when thetemperature is 20° C. or -30° C. and the displacement is 0 mm, 10, or 20mm.

FIG. 25 is a table showing the relationship between driving method andtemperature coefficient.

FIG. 26 is a table showing the circuit output V0 generated when thetemperature is 20° C. or -30° C. and the displacement is 0 mm, 10, or 20mm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The first example of the present invention will be described inreference to the drawings. The displacement sensor of this exampleconsists of a sensing element and a circuit element.

FIG. 1 shoes a sectional view of the sensing element 2 of thedisplacement sensor 1 of this example. The sensing element 2 consists ofa permanent magnet 4, a first Hall element (first magnetoelectricconversion means) 5 placed at a predetermined distance from the N-poleof the permanent magnet 4, and a second Hall element (secondmagnetoelectric conversion means) 6 in contact with the S-pole of thepermanent magnet 4, all contained in a case 3 that is made of anon-magnetic material. The magnetic poles of the permanent magnet 4 canbe reversed.

FIGS. 2 and 3 illustrate the principle upon which the displacement issensed according to eh variations in magnetic flux distribution causedby the displacement of the material to be measured. In FIGS. 2(a) and2(b), the permanent magnet 4, the first Hall element 5, and the secondHall element 6 are identical to that of FIG. 1. The non-magnetic case 3is not shown in FIG. 2. The magnetic material to be measured is a steelwire 7. In this example, the displacement of the material to be measuredoccurs in a lateral direction while maintaining a certain distancevertically from the Hall elements 5 and 6. The displacement sensing canbe performed as well in the case where the displacement occurs in alongitudinal direction.

FIG. 2(a) illustrates the case where the wire is situated on the centerline of the Hall elements 5 and 6, which position is taken as thedisplacement 0. The lines of magnetic force in the drawing show themagnetic flux flowing from the N-pole toward the S-pole while part of itpasses through the wire 7. Since every magnetic material has a magneticpermeability higher than that of air, the magnetic flux is attracted tothe wire 7. FIG. 2(b) illustrates with lines of magnetic force themagnetic flux when the displacement of the wire occurs in a lateraldirection. The magnetic flux is varied as it is attracted to the wire 7that is made of a magnetic material, and the magnetic flux density atthe first Hall element 5 is smaller than that at the displacement 0.

When the driving current is constant, the output of the first Hallelement 5 is given by the expression:

    E=K·B

where E is the output voltage (unit: volt, or V); K is the constant ofthe Hall element determined by the temperature of the externalenvironment, and the driving current; and B is the magnetic flux density(unit: gauss, or G).

Accordingly, the relationship between the displacement of the wire 7 andthe output of the Hall element 5 at a constant temperature is as showngraphically in FIG. 3(a). As the magnetic flux density is reduced by thedisplacement of the wire 7, the output of the first Hall element 5 isreduced.

Meanwhile, as can be seen from FIGS. 2(a) and 2(b), the magnetic fluxdensity at the second Hall element 6 is not varied by the displacementof the wire 7, and therefore, the output of the second Hall element 6 ata constant temperature is constant as shown graphically in FIG. 3(b).

In an experiment using a PrFeB magnet that is 30 mm×30 mm incross-sectional area and has a surface magnetic flux density of 3700 G,and a wire having a diameter of 40 mm, the distance between thepermanent magnet 4 and the first Hall element 5 is 17 mm while thedistance between the first Hall element 5 and the wire 7 is 12 mm, andthe temperature is 20° C. Under such conditions the magnetic fluxdensity at the first Hall element 5 is 1000 G when the displacement ofthe wire is 0 mm, 960 G when the displacement of the wire is 10 mm, and920 G when the displacement of the wire is 20 mm. Thus, in the presentinvention, the displacement ranging from 0 to 20 mm can be sensed in therange of 1000 to 920 G.

In order to measure the displacement of the wire sensitively andaccurately, it is necessary to amplify the output of the first Hallelement 5 to compensate for the difference caused by the displacement,which is equivalent to the margin of 40 G in the above experiment. InFIG. 4, for example, a bias voltage is generated, and then subtractedfrom the amplified output of the first Hall element 5. Reference numeral11 indicates an amplifier for amplifying the output of the first Hallelement 5, reference numeral 12 a bias generating circuit for generatingDC bias voltage depending on the sensitivity of the bias magnetic fieldand the first Hall element, and reference numeral 17 a bias cancellingcircuit for cancelling bias voltage by adding the outputs of theamplifier 11 and the bias generating circuit 12. The output Vo at aconstant temperature is as shown in FIG. 5, where R1=R2=Rf3.

However, the temperature coefficient of the Hall constant of GaAs Hallelements during constant-current drive is -0.06%°C., while thetemperature coefficient of the magnetic flux density of the PrFeB magnetused in this experiment is about -0.11%/°C. The total temperaturecoefficient of the circuit in this experiment is -0.22%/°C. Thefollowing is a case where temperature T1 is 20° C., temperature T2 is-30° C. and the temperature difference between T1 and T2 is 50° C. Inthe circuit shown in FIG. 4, the output of the amplifier 11 is 2.73 Vwhen the temperature is T₁ and the displacement 0 mm. The output V1(V)of the amplifier 11 can be determined by the following formula:

    V1=2.73×10.sup.-3 ×B1×(1-2.2×10.sup.-3 ×(T-20))×f(X)

where B1 is the magnetic flux density (G) at the first Hall element 5 atthe temperature T1; T is the temperature (°C.); f(X) is a function ofthe displacement X (mm). The data compiled in the above experiment show:f(0)=1,f(10)=0.96, f(20)=0.92.

FIG. 20 shows the output Vo in the case where the temperature is T₁ andthe displacement is 0 mm in the bias generating circuit 12 of FIG. 4,and where the output of the bias cancelling circuit 13 is adjusted to 0V while the displacement and temperature are subjected to variations.The temperature coefficient of the whole displacement sensor (with thewire displaced by 10 mm) is +5.28%/°C., and the temperature coefficientof the whole displacement sensor (with the wire displaced by 20 mm) is+2.53%/°C.

As can be seen from the FIG. 20, when only the first Hall element 5 isused, the output variation caused by the 50° C. variation in temperatureis greater than the output variation caused by the 10 mm displacement,which means that the sensor is not accurate enough for a displacementsensor.

In this example, the displacement sensor 1 comprises a circuit 10 shownin FIG. 6 in place of the circuit of FIG. 4. The amplifier 11 and thebias cancelling circuit 17 are identical to that of FIG. 4. Referencenumeral 13 indicates an amplifier for amplifying the output of thesecond Hall element 6, and reference numeral 14 an attenuator forattenuaing the output of the amplifier 13 at a predetermined ratio. Themagnetic flux density at the second Hall element 6 is not varied by thedisplacement of the wire 7. The permanent magnet 4, the first Hallelement 5, and the second Hall element 6 are closely contained in thecase 3 so that the temperature in the sensor is kept constant. In FIG.6, the output of the amplifier 13 is adjusted by feedback resistors orthe attenuator 14 so that the output V0 of the bias cancelling circuitis 0, with T1=20° C. and the displacement 0 mm. Thereby, when T1 is 20°C. and the displacement 0 mm, the output V3 is 2.73 V, which is equal tothe output V1 of the amplifier 11. Accordingly, the output V3 of theamplifier 13 can be determined by the formula:

    V3=2.73×10.sup.-3 ×B2×(1-2.2×10.sup.-3 ×(T-20))

FIG. 21 shows the output Vo in the circuit of FIG. 6, with thetemperature and displacement subjected to variations. The temperaturecoefficient of the whole displacement sensor becomes -0.22%/°C. and thesensing accuracy is dramatically increased, thereby enabling thedisplacement sensor to be put into practical use.

Thus, the temperature characterstics of a displacement sensor comprisinga permanent magnet and Hall elements can be dramatically improved inthis example. Although Hall elements are used in this example, it shouldbe understood that other magnetoelectric conversion elements, forexample, magnetic resistance elements 5' and 6' can also be used asshown in FIG. 7.

The second example of the present invention will be described below.

As shown in FIG. 8, the displacement sensor 1 of this example comprisesa sensing element 2. Contained in a non-magnetic case 3 are a permanentmagnet 4, a first Hall element 5 placed at a predetermined distance fromthe N-pole of the permanent magnet 4, and a second Hall element 6 incontact with the N-pole of the permanent magnet 4. The magnetic poles ofthe permanent magnet can be reversed.

Since the second Hall element 6 is in contact with the N-pole of thepermanent magnet 4, the magnetic flux density is not varied by thedisplacement of the material to be measured, just as in the firstexample. Therefore, the displacement of the material to be measured 7,as in the first example, can be sensed by the circuit 10 shown in FIG.6, regardless of variations in temperature. In this example, too, themagnetoelectric conversion elements may be magnetic resistance elements.

The third example of the present invention will be described inreference to the drawings. This example is aimed at further improvingthe accuracy of the displacement sensor of the first example.

The sensing element 2 of the displacement sensor 1 of this example isconstituted in the same manner as in FIG. 1 showing the first example.The magnetic flux density at the first Hall element 5 is 1000 G when thedisplacement of the wire is 0 mm, 960 G when the displacement is 10 mm,and 920 G when the displacement is 20 mm. The temperature coefficient ofthe sensing element 2 is -0.22%/°C.

In the first example, as shown in FIG. 21, when the displacement is 10mm, the output is varied 10% by the 50° C. temperature variation, whichmeans that the sensor is not accurate enough for a displacement sensorthat is required to be extremely accurate.

In this example, to further improve the accuracy of the wholedisplacement sensor 1, temperature sensing resistors having the oppositetemperature coefficient to that of the permanent manget 4 and the firstand second Hall elements 5 and 6 are employed as feedback resistors Rf1,Rf2, and Rf1', Rf2' in the circuit means 10 as shown in FIG. 9. Sincethe total temperature coefficient of the permanent magnet 4 and the Hallelements 5 and 6 is -0.22%/°C., the temperature resistors having atemperature coefficient of +0.22%/°C. and the same resistance is used asthe feedback resistors Rf1, Rf2, and Rf1', Rf2' in the circuit of FIG.9, where R1=R2=Rf3. In the circuit of this example, the output of theamplifier 11 is 2.73 V when the temperature T1 is 20° C. and thedisplacement is 10 mm. The output V1(V) of the amplifier 11 can bedetermined by the following formula:

    V1=2.73×10.sup.-3 ×B1×(1-2.2×10.sup.-3 ×(T-20))×(1+2.2×10.sup.-3 ×(T-20))×f(X)

In the circuit shown in FIG. 9, when the temperature is T1 and thedisplacement is 0 mm, the voltage V2 of the amplifier 13 which isadjusted by the attenuator 14 is 2.73 V, which is equal to the output V1of the amplifier 11 for adjusting the output V0 of the bias cancelingcircuit 17 to 0 V. Since the magnetic flux density at the second Hallelement 6 is not varied by the displacement of the wire 7, the outputV3(V) can be determined by the formula:

    V3=-2.73×(1-2.22×10.sup.-3 ×(T-20))×(1+2.2×10.sup.-3 ×(T-20))

FIG. 22 shows the output V0 when the displacment and temperature aresubjected to variations. The temperature coefficient of the wholedisplacment sensor is +0.02%/°C. (the displacement is 10 mm or 20 mm).

In this example, the output variation by the 50° C. temperaturevariation is not caused even when the displacement is 20 mm, thusenabling accurate sensing of the displacement.

Although this example has been described in its preferred form with theuse of Hall elements, it should be understood that other magnetoelectricconversion means, such as magnetic resistance elements, can be used inplace of the Hall elements. While the temperature sensing resistors areused in this example as the feedback resistors for the amplifier 11 and13 to compensate for temperature variations, the temperature sensingresistors can also be used as resistors at the input stages of theamplifiers 11 and 13, or as feedback resistors for the amplifier 17, oras partial pressure resistors at the input stages of R1, R2 and the Hallelements, without detracting the expected effects. Further, thepolarities of the permanent magnet 4 can be reversed even if the firstHall element 5 is located at a predetermined distance from the N-pole ofthe permanent magnet 4 and the second Hall element 6 is located incontact with the N-pole of the permanent magnet 4.

Described below is the fourth example of the present invention. In thisexample, the temperature characteristics of the displacement sensor isfurther improved by arbitrarily adjusting the temperature coefficient ofthe permanent magnet and the Hall elements.

As shown in FIG. 10, the displacement sensor 1 of this example comprisesa sensing element 2 which consists of a permanent magnet 4, a first Hallelement 5, and a second Hall element 6, all of which are contained in acase 3. A circuit means 10 is attached to a side of the case 3. Thestructure of the circuit means 10 is identical to that of FIG. 6.

In this example, the temperature coefficient of the permanent magnet 4and the first and second Hall elements 5 and 6 is adjusted so that thecoefficient of the displacement sensor as a whole approaches zero. Forinstance, when the temperature coefficient of the permanent magnet 4 andthe first Hall element 5 is equal to that of the permanent magnet 4 andthe second Hall element 6, the temperature coefficient of thedisplacement sensor is negative. However, it is possible to minimize thetemperature coefficient of the displacement sensor to zero by making theHall element 5 and 6 have temperature coefficient different from eachother with the permanent magnet 4 used in common. FIG. 23 shows therelationship between V1, V3, and V0, where the displacement is 10 mm.The temperature coefficient of the Hall elements 5 and 6 are arbitrarilyadjusted by selecting a combination of the outputs of V1 and V3 so thatthe output V0 becomes zero, taking into account the relationship betweenthe output V0 and the outputs V1 and V3.

In this example, elements each having a temperature coefficient that isvaried by the magnetic flux density, for example, InAs elements, areused as the first and second Hall elements 5 and 6. The temperaturecoefficients of the Hall elements 5 and 6 are arbitrarily adjusted sothat they differ from each other.

With the InAs Hall elements, the temperature coefficient (%/°C.) of thepermanent magnet 4 plus the Hall elements is varied depending on themagnetic flux density (G) of the permanent magnet 4, as shown in FIG.11.

In this example, with a wire 7 having a diameter of 28 mm used as thematerial to be measured, the relationship between the magnetic fluxdensity (G) and the distance (mm) from the surface of the magnet to theHall element can be shown in FIG. 12. When the distance between theupper end of the permanent magnet 4 and the first Hall element is 7 mmand the distance between the first Hall element 5 and the outer surfaceof the wire 7 is 5 mm, the temperature coefficient of the permanentmagnet 4 and the first Hall element 5 becomes -0.29%/°C. while thetemperature coefficient of the permanent magnet 4 and the second Hallelement 6 becomes -0.28%/°C. and the displacement is 0 mm, the output V1of the amplifier 11 is 2.73 V. The output V1 of the amplifier 11 can bedetermined by the formula:

    V1=2.73×10.sup.-3 ×B1×(1-2.9×10.sup.-3 ×(T-20))×f(X)

When the temperature is T1 and the displacement is 0 mm, the output V3of the amplifier 13 which is adjusted by the attenuator 14 is 2.73 V,which is equal to the output V1 of the amplifier 11 for adjusting theoutput V0 of the bias cancelling circuit 17 to 0 V. Since the magneticflux density at the second Hall element 6 is not varied by thedisplacement of the wire 7, the output V3(V) of the amplifier 13 can bedetermined by the formula:

    V3=-2.73×(1-2.8×10.sup.-3 ×(T-20))

FIG. 24 shows the output V0 when the displacement and temperature aresubjected to variations. The temperature coefficient of the wholedisplacment sensor is -0.04%/°C. when the displacement is 10 mm, and thetemperature coefficient is -0.17%/°C. when the displacement is 20 mm.

In this example, the output variation by the 50° C. temperaturevariation is very small even when the displacement is 20 mm, thusenabling accurate sensing of the displacement.

Although the second Hall element 6 is disposed in contact with theS-pole of the permanent magnet 4 in this example, the second Hallelement 6 may be in contact with the N-pole of the permanent magnet 4 asshown in FIG. 13.

Next, the fifth example of the present invention will be described. Justas in the fourth example described before, the temperaturecharacteristics of the displacement sensor is minimized by adjusting thetemperature coefficient of the permanent magnet and the Hall elements.Two Hall elements having different temperature coefficients from eachother are used as the first and second Hall elements. The sensingelement 2 and the circuit means 10 are formed in the same manner as inthe fourth example.

In this example, the two Hall elements having different temperaturecoefficients from each other are used as the first and second Hallelements 5 and 6. The Hall elements 5 and 6 are arbitrarily employed sothat the temperature coefficient of the displacement sensor 1 approacheszero. The temperature coefficient of each of the Hall elements 5 and 6is determined depending on which driving method is suitable for eachHall element.

As for the driving method for the Hall elements 5 and 6, FIG. 14illustrates a constant-voltage driving circuit 18 and FIG. 15 shows aconstant-current driving circuit 19. In the drawings, Tr indicates atransistor, E indicates a DC power source, R1 indicates a partialpressure resistor, D1 and D2 indicate bias diodes, R2 indicates anemitter resistor, and 5 and 6 indicate Hall elements.

As the Hall elements 5 and 6, a GaAs element and an InSb element areemployed. Each Hall element is combined with the driving circuit 18 or19 so that the temperature coefficient can be arbitrarily determined asshown in FIG. 25.

Thus, by using two different Hall elements and combining them with adriving circuit, the temperature coefficient of the displacement sensorcan be minimized close to zero.

The sixth example of the present invention will be described below. Inthis example, the temperature of the displacement sensor 1 itself iskept constant so as to be independent of the temperature of the externalenvironment.

As shown in FIG. 16, contained in a case 21 are a sensing element 2(consisting of a permanent magnet 4, first and second Hall elements 5and 6) of the displacement sensor 1, a circuit means 10, a temperaturesensor 22 for sensing the temperature of the inside of the case 21, aheater 23 for heating the inside of the case 21, and a temperaturecontroller 24 for controlling the temperature of the inside of the case21 by driving the heator 23 so as to obtain a predetermind temperature.

Since the permanent magnet 4 and the first and second Hall elements 5and 6 are constantly maintained at a predetermined temperature, thepermanent magnet 4 and the first and second Hall elements 5 and 6 arenot adversely affected by the temperature variations of the externalenviornment, whereby highly reliable sensing of the displacement can beperformed using the circuit of the first example shown in FIG. 6.

The inventors of the present invention have conducted some experimentson the output V0 and the results are shown in FIG. 26. The temperaturecoefficient of the displacement sensor 1 is 0%/°C. (the displacement is10 mm).

The seventh example of the present invention will be described. Thisexample is aimed at improving the response of the output. Since thepermanent magnet 4 and the first and second Hall elements 5 and 6 areall contained in the case 3 as shown in FIGS. 1, 6, 10, and 13 of theforegoing examples, the thermal conductivity depends on the materialused for the case 3 and the temperature of the external environment.

In this example, the case 3 is made of a material having a high thermalconductivity k (k>10 (W·m⁻¹ ·k⁻¹), such as aluminum. FIG. 17 shows thecharacteristics of the variation ratios of the outputs V1 and V2 of thepermanent magnet 4 and the first and second Hall elements 5 and 6 whenthe case 3 is made of aluminum. FIG. 18 shows the characteristics of thevariation ratios of the outputs V1 and V2 when the case 3 is made offluorine contained resin which has a low thermal conductivity.

As can be seen from both drawings, in the case of aluminum, there islittle difference in output response caused by variations in position,because of the rapid response to the variation ratios of the outputs V1and V2. On the other hand, in the case of fluorine contained resin,there is a significant difference in output response, due to the slowresponse to the variation ratios of the outputs V1 and V2.

FIG. 19 shows the characteristics of the output V0 of the displacementsensor with the lapse of time in both cases where the case 3 is made ofaluminum and where the case 3 is made of fluorine contained resin. Whenfluorine contained resin is used, the response is slowed in both Hallelements 5 and 6. With the case 3 made of aluminum in accordance withthis example, the response is hardly slowed.

Thus, by forming a case of a material having a high thermal conductivityin accordance with this example, it is possible to obtain a sensor whichhardly causes a lag in the response due to the temperature variations ofthe Hall elements.

Although the examples of the present invention have been described inthe preferred form with the use of the permanent magnet 4, one morepermanent magnet of the same type can be used and the outputs of thefirst and second magnetoelectric conversion means are generated by therespective permanent magnets. In such case, electromagnets can also beused in place of permanent magnets.

INDUSTRIAL FIELD OF THE INVENTION

With the simple structure in which a magnet and magnetoelectricconversion means which are generally used are employed, the displacementof a magnetic material to be measured can be accurately sensed over awide temperature range. A displacement sensor of such structure can beused outdoors or even in a place where the temperature is low and variesin a wide range, for example, in highlands or in a harshly cold place.The displacement of a magnetic material such as a wire can be accuratelysensed by the displacement sensor of the present invention even in aharsh environment as descrived above. Therefore, the displacement sensorof the present invention is desirable when high precision sensing isneeded.

We claim:
 1. A displacement sensor for sensing displacement of a magnetic material to be measured, comprising:first magnetoelectric conversion means located in a place where a magnetic flux density is varied by displacement of said magnetic material to be measured, second magnetoelectric conversion means of the same type as that of said first magnetoelectric conversion means located in a place where a magnetic flux density is not varied by the displacement of said magnetic material to be measured, and circuit means connected to said first and second magnetoelectric conversion means for adding outputs of said two magnetoelectric conversion means at a ratio of V20/V10 to provide a result of V2-[V1×(V20/V10)], wherein V1 and V2 are outputs from said first and second magnetoelectric conversion means, respectively, and V10 and V20 are outputs from said first and second magnetoelectric conversion means, respectively, for a displacement of 0 at a reference temperature.
 2. A displacement sensor according to claim 1, wherein said reference temperature is 20° C.
 3. A displacement sensor according to claim 1, wherein said first and second magnetoelectric conversion means, and said circuit means are all contained in a nonmagnetic case.
 4. A displacement sensor according to claim 3, wherein said case has a heat insulating structure so that the temperature in said case is kept constant.
 5. A displacement sensor according to claim 3, wherein said case is made of a material having a high thermal conductivity.
 6. A displacement sensor for sensing displacement of a magnetic material to be measured, comprising:first magnetoelectric conversion means located in a place where a magnetic flux density is varied by displacement of said magnetic material to be measured, second magnetoelectric conversion means of the same type as that of said first magnetoelectric conversion means located in a place where a magnetic flux density is not varied by the displacement of said magnetic material to be measured, first circuit means provided in said first and second magnetoelectric conversion means and having a temperature coefficient of a polarity opposite to that of said magnet and said magnetoelectric conversion means so as to compensate for caused by temperature variations, and second circuit means connected to said first and second magnetoelectric conversion means for adding the outputs of said two magnetoelectric conversion means at a ratio of V20/V10 to provide a result of V2-[V1×(V20/V10)], wherein V1 and V2 are outputs from said first and second magnetoelectric conversion means, respectively, and V10 and V20 are outputs from said first and second magnetoelectric conversion means, respectively, for a displacement of 0 at a reference temperature.
 7. A displacement sensor according to claim 1 or 6, wherein said magnetoelectric conversion means are Hall elements or magnetic resistance elements.
 8. A displacement sensor according to claim 6, wherein said reference temperature is 20° C.
 9. A displacement sensor for sensing displacement of a magnetic material to be measured, comprising:first magnetoelectric conversion means located in a place where a magnetic flux density is varied by displacement of said magnetic material to be measured, second magnetoelectric conversion means of the same type as that of said first magnetoelectric conversion means located in a place where a magnetic flux density is not varied by the displacement of said magnetic material to be measured, circuit means connected to said first and second magnetoelectric conversion means for adding the outputs of said two magnetoelectric conversion means at a ratio of V20/V10 to provide a result of V2-[V1×(V20/V10)], wherein V1 and V2 are outputs from said first and second magnetoelectric conversion means, respectively, and V10 and V20 are outputs from said first and second magnetoelectric conversion means, respectively, for a displacement of 0 at a reference temperature, and means for adjusting a temperature coefficient of said first magnetoelectric conversion means so as to differ from a temperature coefficient of said second magnetoelectric conversion means thereby providing accurate displacement reading in a wide range of temperature variations.
 10. A displacement sensor according to claim 9, wherein said reference temperature is 20° C.
 11. A displacement sensor according to claim 9, wherein said first and second magnetoelectric conversion means are composed of elements each having a temperature coefficient which varies depending on the magnetic flux density.
 12. A displacement sensor according to claim 9, wherein said first and second magnetoelectric conversion means are InAs elements.
 13. A displacement sensor according to claim 9, wherein temperature coefficients are adjusted by combining the driving methods used for said first and second magnetoelectric conversion means. 